1. Field of the Invention
This invention is directed to a semiconductor device manufacturing process. This invention is also directed to a metallization scheme and fabrication process for forming an Ohmic contact on a wide bandgap semiconductor material. This invention is further directed to a metallization scheme and fabrication process for forming a low resistance, thermally stable Ohmic contact on n-type silicon carbide (SiC) using the post-deposition annealed (950° C.–1000° C.) composite metallization scheme Platinum/Titanium/Tungsten silicide/Nickel (Pt/Ti/WSi/Ni)
2. Discussion of Relevant Arts
Silicon carbide (SiC) is an excellent candidate for high temperature and high power device applications because of its combination of electronic and thermal properties, namely, wide energy bandgap, high electric breakdown field, large saturated electron drift velocity and high thermal conductivity (See Philip G. Neudeck, J of Electronic Materials. 24, 283.(1995); S. J. Pearton. et al, Electrochemical Society Proc. 97-1,138(1997); M. R. Melloch et al., M R S Bulletin 23, 42 (1997); T. P. Chow et al, IEEE Trans. Electron Devices 41, 1481 (1994); and T. P. Chow et al., Mat. Res. Soc. Proc. 423, 9 (1996)).
Based on these properties, devices fabricated from SiC deliver superior performance over existing devices. Rapid advances in the growth, doping and processing of SiC have led to the realization of several electronic and photonic devices including fast recovery high voltage diodes, MOSFETs, MESFETs, SITs, JFETs, UV photodiodes, SiC bipolar devices, BJTs and HBTs. The wide bandgap and high thermal conductivity are attractive for high temperature digital integrated circuits and nonvolatile solid-state memories. Although progress with SiC based electronic devices has been encouraging, there are significant challenges to overcome. This is particularly relevant in the areas of physical and chemical development, electrical stability and reliable multilevel metallization technology capable of high packing density.
An important requirement of all device technologies is the development of electrical contacts with low specific contact resistance and high stability and long term reliability. Ohmic contacts with low specific contact resistance and good thermal stability are necessary to obtain optimum performance from high temperature, high power, and high frequency devices.
As the device dimensions continue to decrease, much more stringent requirements are being placed on the material, processing and electrical performance of low resistance Ohmic contacts. Metallization of wide bandgap semiconductors (SiC) is complicated, particularly because of their high surface reactivity, low doping concentrations, and high density of interface states. Most SiC based electronic devices which cannot sustain long-term operation at an elevated temperature/power level suffer deterioration of their metal/SiC contacts (See L. M. Porter et al., Mat. Sci. and Eng. B34, 83 (1995)). Thus, an important concern in the development of SiC devices is the formation of low resistance Ohmic contacts with good thermal, chemical and mechanical stability.
To date, many metallizations, namely Ni, Al/Ni/Al, Cr, Al, Au—Ta, TaSi2, W, Ta, Ti, TiW, TiC, Ti/Au, TiSi2, Co, Hf, Re, and WSi have been investigated for Ohmic contacts to n-SiC (See J. Crofton et al, Phys. Stat. Sol. 202, 581 (1997); and M. W. Cole et al., Electrochemical Society Proc. 28, 71 (1998)). Industry standards have deemed Nickel Ohmic contacts to be the preferred standard contacts for SiC devices. Nickel Ohmic contacts to n-SiC possess a low specific contact resistance (pc) less than 5.0×10−6 Ω-cm2, and good physical thermal stability at temperatures up to 500° C. for ˜100 h. In addition, from the point of electrical integrity, nickel Ohmic contacts to n-SiC are reproducible (See Crofton above; See also Crofton et al., J. Appl. Phys. 77, 1317 (1995); Crofton et al., Proc. Of the Fourth Int'l. High Temperature Electronics Conference, 4, 84 (1998); Marinova et al., Materials Science and Engineering B46, 223 (1997)).
Ni-nSiC Ohmic contacts are known to be formed by depositing pure metallic nickel on the n-SiC substrate. This intermediate (Ni—SiC) is then furnace annealed at temperatures of about 950° C. for 2 to 5 min or rapid thermal annealed (RTA) at temperatures of about 950 to 1000° C. for 30 to 60 seconds. Annealing results in the formation of the intermetallic phase Ni-silicide (N2Si) overlying the SiC substrate material (See Crofton et al, Phys. Stat. Sol., 202; Crofton et al., J. Appl. Phys., 77; Crofton et al., Proc. Of the Fourth Int'l High Temperature Electronics Conference, 4; and Marinova et al., Materials Science and Engineering B46, above; see also Luckowski et al, Mat. Res. Soc. Symp. Proc. 423, 119 (1996); Adams et al., Proc. Of the Second Int'l High Temperature Electronics Conference, 2, 9 (1994); Goesmann et al, Materials Science and Engineering B46, 357 (1997); Porter et al., Mater. Res. 10, 668 (1995); and Waldrop et al., Appl. Phys. Lett. 62, 2685 (1993)).
The resulting Ohmic contact composition is represented by the chemical formula Ni2Si—SiC. Forming Ni2Si by annealing Ni—SiC at 950 to 1000° C. has been reported to cause a lower resistance of the initial Ni—SiC contact. Therefore, it is actually this Ni2Si—SiC composition and not pure Ni intermediate contact that displays the low specific contact resistance reported above.
The high temperature annealing process used to form these Ni2Si—SiC Ohmic contacts have resulted in several undesirable features which cause device unreliability and ultimate device failure (See Crofton et al., Phys. Stat. Sol., 202; Crofton et al., Proc. Of the Fourth Int'l. High Temperature Electronics Conference, 4; and Marinova et al., Materials Science and Engineering B46, above; see also Getto et al, Material Science and Engineering B61–62, 270 (1999)).
The undesirable features of these Ni2Si—SiC Ohmic contacts include:
1. Substantial broadening of the contact layer thickness or metal-SiC interface expansion.
The increase in contact thickness via consumption of the SiC substrate is due to the high reactivity of Ni with Si to form Ni-silicide leaving behind both voids and unreacted carbon. Annealing the Ni—SiC contact results in a contact thickness increase of ≦100%. Such an increase in contact thickness makes the annealed Ni—SiC Ohmic contact incompatible for device designs which possess shallow p-n junctions.
2. A rough interface morphology heavily laden with Kirdendall voids.
The voids resulting from the high reactivity of Ni with Si at the interface will cause internal stress and possible delamination of the contact layer, which will compromise device reliability. The internal stress and contact delamination will be significantly amplified under the extreme thermal and electrical stresses typical of the power device operational environment and will ultimately result in device failure. The rough interface morphology makes the annealed Ni—SiC Ohmic contact unsuitable for device designs which possess shallow p-n junctions. Thus loss of a sharp interface will compromise device designs which posses shallow p-n junctions.
3. Carbon segregation at the metal SiC interface and/or throughout the metal layer.
Though x-ray photoelectron spectroscopy (XPS) analysis of the annealed contact, it is known that carbon is in the graphite state and that Si is bonded predominantly to Ni resulting in Ni-Silicide formation. Dissociation of SiC to Si and C in the presence of Ni atoms is possible at temperatures above 400° C. Thus the dissociation of SiC at the Ni/SiC interface to Si and C is due to the reactivity of Ni. Carbon inclusions at the metal-SiC interface and/or within the contact layer are considered a potential source of electrical instability, especially after prolonged operation of the devices at high temperatures. At elevated temperatures, redistribution of carbon inclusion occurs, resulting in significant degradation of the contact's electrical and microstructural properties.
4. Substantial roughening of the contact surface (on the order of tens of nanometers).
For many device applications, Ohmic contacts must be wire bonded to a die package. A rough surface morphology will most likely cause wire bonding difficulty and/or failure under the extreme thermal fatigue during high power and high temperature device operation. Additionally, rough surface morphology is not desirable for high current applications because it causes non-uniformity of current flow. In addition, contact surface roughness results in residual material stresses that may induce SiC polytype changes. Alteration of the SiC polytype also alters the electrical properties. For example, a polytype change from 4H to 6H degrades the electron mobility of the SiC and degrades the device.
Therefore, even though Ni contacts possess excellent electrical properties, the above mentioned features will lead to device reliability problems and ultimately cause device failure via contact degradation and/or wire bond failure after exposure to long term high power and high temperature device operational stresses.